1) Field of the Invention
The present invention relates to an X-ray analyzing apparatus, and particularly to an apparatus arranged such that an X-ray detector and a cooling means for cooling the detector are sealed in a vacuum container and the characteristic X-ray generated from a sample to be analyzed is detected through an X-ray transmitting window provided on the vacuum container.
2) Related Art
FIG. 1 is a schematic view showing construction of a conventional X-ray analyzing apparatus. As shown in FIG. 1, the conventional X-ray analyzing apparatus comprises an excitation ray generator 11, a sample chamber 13 for containing a sample 12 and an X-ray detector 15 which is sealed in a vacuum container 14. A primary ray 16 which is generated from the excitation ray generator 11 is applied onto the sample 12 to allow that a characteristic X-ray 17 is generated from the sample. The generated characteristic X-ray 17 is then detected by the X-ray detector 15. The signal detected by the X-ray detector 15 is converted into an electric signal to conduct a qualitative/quantitative analysis of the object substance contained in the sample.
As the excitation ray generator 11, an ion source, an electron-beam generator, an X-ray generator, or a radiation source is preferably used. As the X-ray detector 15, a Silicon semiconductor element is preferred. The semiconductor detector 15 is connected to a heat conducting rod 19 and cooled by a cooling means 18 via the rod 19.
Cooling the detector reduces thermal noise and improves the signal to noise ratio so that an analysis with a high accuracy can be realized. The cooling means 18 is preferably a container which includes a refrigerant, such as liquid nitrogen, or a freezing device. In order to cool the semiconductor detector 15 effectively, the detector 15 and the cooling means 18 are sealed in a vacuum container 14. To maintain the vacuum condition in the vacuum container 14 in which the semiconductor detector 15 is contained and to introduce the characteristic X-ray 17 generated from the sample 12 into the semiconductor detector 15, an extremely thin X-ray transmitting window 20, made of Beryllium or an organic film, is provided between the sample chamber 13 and the vacuum container 18. The characteristic X-ray generated from the sample 12 is detected via the window 20.
In case a light element having a small atomic number is analyzed in such an X-ray analyzing apparatus, the attenuation of X-ray through the X-ray path of the apparatus should be as small as possible because the X-ray to be analyzed is apt to be easily attenuated and the energy level of the X-ray is low. In order to avoid the attenuation of the X-ray, the sample chamber 13 is generally arranged as a vacuum.
However, when a highly liquid, or highly volatile or a biological sample having a high liquid-water content, is analyzed, the sample chamber 13 is not arranged to be vacuumed because doing so would cause evaporation of the sample. A diaphragm between the sample and the sample chamber has been suggested to prevent the evaporation. However, when the sample chamber is arranged to be vacuumed, it does not help to make the diaphragm thick. If a thick diaphragm wall is provided between the sample and the sample chamber, the characteristic X-ray is attenuated by the diaphragm itself. Further the cost of manufacturing the apparatus becomes high because the material for such diaphragm is very expensive.
It has been suggested to purge a gas in the sample chamber 13, which does not prevent the transmission of the X-ray and does not generate the other characteristic X-rays which disturb the analysis of the object substance. Gases having a small atomic number, such as Hydrogen and Helium, do not prevent the transmission of X-rays. The characteristic X-rays generated from these gases have only small energy. These gases therefore can be preferably used to purge the sample chamber because the analyzing result is not effected by the characteristic X-rays generated from the gases. Helium gas is able to be treated safely and easily so that it is particularly favorable as the gas for purging the sample chamber.
In this manner, by purging the sample chamber 13 with a gas such as Helium, attenuation of the characteristic X-ray to be detected can be prevented in the sample chamber and high measurement accuracy can be retained, while the sample can be protected from evaporation. However, the conventional apparatus shown in FIG. 1 still has a drawback in that the gas purged into the sample chamber 13 leaks into the vacuum container 14. Leakage occurs because the apparatus is arranged such that the gas is directly contacted with the X-ray transmitting window 20 (a film made of Beryllium) by which the vacuum container 14 is separated from the sample chamber 13 and because Hydrogen or Helium is composed of such small molecules that they can pass through the lattice of the Beryllium atom. When the gas comes into the vacuum container 14, the vacuum condition of the vacuum container 14 deteriorates so that the heat insulating effect for the semiconductor X-ray detector 15 is gradually loosened. That is to say, the vacuum condition in the vacuum container 14 is deteriorated by the gas purging which should be done to protect the sample from evaporation. As a result, the consuming amount of two liquid nitrogen for cooling the semiconductor X-ray detector 15 increases in accordance with the deterioration of the vacuum condition of the vacuum container 14. If the consuming amount of utility such as the liquid nitrogen is increased, the liquid nitrogen supply period becomes quicker, leading to various problems would be caused such that the running cost of the apparatus increases and prediction for the period for exchanging the utility becomes difficult.
Further, since high electric power is applied to the semiconductor X-ray detector 15, if the vacuum condition of the vacuum container 14 deteriorates, a discharge is generated in the container so that performance of the semiconductor detector 15 itself is aggregated and leading to the possibility that the detector 15 would be broken.